JP2897515B2 - Voltage-current converter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a voltage / current conversion circuit.

[0002]

2. Description of the Related Art A conventional voltage-current conversion circuit is generally used for supplying a current having a magnitude proportional to an input voltage to a predetermined load. FIG. 3 is a circuit diagram of a conventional well-known voltage-current conversion circuit. As shown in FIG.
The PNP transistor 18 and the resistor 19 correspond to the supplied power supply voltage V _cc , the input terminal 55 and the output terminal 56.
, An NPN transistor 20, diodes 22 and 23, and a constant current source 21.

In FIG. 3, the collector of an NPN transistor 20 is connected to a power supply voltage _Vcc , and the base is
The diodes 22 and 23 are clamped to a voltage level shifted from the ground potential level by a forward voltage of the two diodes. Moreover, the base of the PNP transistor 18 is supplied with the input voltage V _i through an input terminal 55, the emitter and the NPN transistor 20
A resistor 19 (resistance value R) is connected between the emitters and the diodes 22 and 23 are biased by a constant current source 21 (current value I).

Now, a PNP transistor 18 and an NPN
If the current amplification factor β of the transistor 20 is sufficiently large and the base-emitter voltage V _BE is equal to the forward voltage V _Df of the diodes 22 and 23,
In case of a V _BE ┃ = V _Df, the output current at the input voltage V _i and the output terminal 56 of the input terminal 55 I
_The following relationship holds between _o and the resistance value R of the resistor 19.

I _o = (V _i + 2V _Df −2 BEV _BE ┃) / R = V _i / R (1) That is, the output current is the input voltage V _i. Is obtained in proportion to the current value _Io .

[0006]

In the above-described conventional voltage-current converter, the base-emitter voltage V _BE of the PNP transistor 18 and the NPN transistor 20 and the forward voltage V _Df of the diodes 22 and 23 are provided.
Are set equal, but in practice these voltage values are not completely equal, and are generally given by the following equations, respectively.

{V _BE } = A · ln (I _o / I _s ) (2) V _Df = A · ln (I / I _s ) ·············· in the ... (3) the above equation, I _s the PNP transistor 18, NPN
A represents a saturation current value of the transistor 21, the diodes 22 and 23, and A represents a constant value represented by kT / q. Needless to say, k is a Portzman constant, T is an absolute temperature, and q is an electron charge.

Therefore, as is apparent from the equations (2) and (3), except for the case where I _o = I, {V _BE ┃ ≠ V _Df
Due to the mismatch between V _BE and V _{Df, in} the conventional voltage-current conversion circuit shown in FIG. 3, the relationship of I _o = V _i / R is not established, and the output current proportional to the input voltage V _i is increased. I can't get it. Relationship between the input voltage V _i and the output current I _o in this case, the above equation (1) is given by the following equation (2) and (3) below.

I _o = [V _i + 2A · ln (I / I _o )] / R (4) FIG. 4 shows the voltage-current conversion characteristics of the present invention and the conventional voltage-current conversion circuit. FIG. In FIG. 4, the horizontal axis represents the input voltage V _i (V: volt).
And the vertical axis represents the output current I _o (mA: milliampere)
, And the characteristic of the conventional example is indicated by a broken line 102. Obviously, the voltage-current conversion gain in response to the input level has changed in the input voltage V _i, not good characteristic is obtained in the linearity.

That is, the conventional voltage-current conversion circuit has a drawback that good voltage-current conversion characteristics cannot be obtained.

[0011]

A voltage-current conversion circuit according to the present invention comprises a first PNP transistor having a collector connected to a current output terminal and a base connected to a voltage input terminal;
An emitter is connected to the emitter of the first PNP transistor via a predetermined resistor, and the potential of the base is changed from the ground potential by an amount equivalent to the forward voltage of the diode via a diode whose cathode is connected to the base. A first NPN transistor that is clamped by the shifted voltage, an input terminal connected to the collector of the first NPN transistor, and an output terminal connected to the base of the first NPN transistor; A current mirror circuit for supplying a current having a current value equal to the output current flowing through the NPN transistor to the diode as an output current;
It is comprised including.

The current mirror circuit has a collector and a base commonly connected to a collector of the first NPN transistor, an emitter connected to a power supply voltage, and a collector connected to the first NPN transistor.
A third PNP transistor connected to the base of the PN transistor, the base connected to the base of the second PNP transistor, and the emitter connected to the power supply voltage. A mirror circuit includes a second PNP transistor having a collector connected to the collector of the first NPN transistor and an emitter connected to the power supply voltage, and a collector connected to the first NPN transistor.
A third PNP transistor having a base connected to the base of the PN transistor, a base connected to the collector of the first NPN transistor, and an emitter connected to the base of the second PNP transistor; Fourth PN transistor commonly connected to the bases of two PNP transistors and having an emitter connected to the power supply voltage.
And a P transistor.

[0013]

Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a first embodiment of the present invention. As shown in FIG. 1, in the present embodiment, the supplied power supply voltage V _cc , the input terminal 51 and the output terminal 52
, The PNP transistor 1, the resistor 2, and the NP
The circuit includes an N transistor 3, a current mirror circuit 6 including PNP transistors 4 and 5, and diodes 7 and 8.

In FIG. 1, the base of NPN transistor 3 is clamped by diodes 7 and 8 to a voltage level shifted from the ground potential level by the forward voltage of the two diodes. Also, PNP
The base of the transistor 1 is the input voltage V _i through an input terminal 51 is supplied, an emitter and a NPN transistor 3
A resistor 2 (resistance value R) is connected between the emitters and the connection relationship and operation thereof are the same as those in the above-described conventional example. This embodiment is different from the conventional example in that the collector of the NPN transistor 3 is connected to the collector of a PNP transistor forming the output terminal of the current mirror circuit 6, and the cathode of the diode 8 is connected to the output terminal of the current mirror circuit. Is connected to the collector of the PNP transistor 5.

In FIG. 1, the output current I _o output via the output terminal 52 is the collector current I _c1 of the PNP transistor 1 itself (I _o = I _c1 ) and passes through the function of the current mirror circuit 6. , Clamping diode 7
And 8 are biased by a current whose current value is exactly equal to the collector current of the NPN transistor 3, so that the following expression is satisfied in all input voltage level ranges corresponding to the above expression (4).

I _o = [V _i + 2A · ln (I _o / I _c1 )] / R = V _i / R ……………………… (5) In FIG. The solid line 101 shows the voltage-current conversion characteristic in the present embodiment. Obviously, the voltage-current conversion gain is constant (1 / R) regardless of the input voltage, that is, the output current value. You can see that there is.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention. As shown in FIG. 2, in the present embodiment, the supplied power supply voltage V _cc , the input terminal 53 and the output terminal 5
4, a PNP transistor 9, a resistor 10,
It comprises a current mirror circuit 15 including PNP transistors 12, 13 and 14, and diodes 16 and 17.

In this embodiment, a Wilson-type current mirror circuit is used as the current mirror circuit 15, and the PNP forming the current mirror circuit 15 is used.
It is characterized in that the variation of the Miller coefficient due to the variation of the current amplification factor β in the transistors 12, 13 and 14 is reduced, so that even if the current amplification factor varies, the influence on the voltage-to-current conversion gain is minimal. There is an advantage that it is suppressed.

[0020]

As described above, according to the present invention, by providing a current mirror circuit which acts to bias the bias to the clamping diode by a current having a current value equal to the output current, the present invention provides the entire level of the input voltage. Within the range, there is an effect that the voltage-current conversion gain can be kept constant.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a conventional example.

FIG. 4 is a diagram illustrating voltage-current conversion characteristics of the voltage-current conversion circuit of the present invention and a conventional example.

[Explanation of symbols]

1, 4, 5, 9, 12 to 14, 18 PNP transistor 2, 10, 19 Resistor 3, 11, 20 NPN transistor 6, 15 Current mirror circuit 7, 8, 16, 17, 22, 22, Diode 21 Constant current source

Claims (3)

(57) [Claims] A first PNP transistor having a collector connected to the current output terminal and a base connected to the voltage input terminal; an emitter connected to the emitter of the first PNP transistor via a predetermined resistor; A first NPN transistor in which a potential of the base is clamped by a voltage shifted from a ground potential by an amount corresponding to a forward voltage of the diode via a diode having a cathode connected to the base; The output terminal is connected to the collector of the first NPN transistor, the output terminal is connected to the base of the first NPN transistor, and a current having a current value equal to the output current flowing through the first NPN transistor is supplied to the diode as an output current. And a current mirror circuit, comprising: 2. A current mirror circuit comprising: a second PNP transistor having a collector and a base commonly connected to a collector of the first NPN transistor and an emitter connected to a power supply voltage; and a collector connected to the first NPN transistor. The voltage-current converter according to claim 1, further comprising: a third PNP transistor connected to a base of the transistor, a base connected to a base of the second PNP transistor, and an emitter connected to the power supply voltage. circuit. 3. The current mirror circuit according to claim 2, wherein a collector is connected to a collector of the first NPN transistor and an emitter is connected to a power supply voltage.
A third transistor having a P transistor, a collector connected to the base of the first NPN transistor, a base connected to the collector of the first NPN transistor, and an emitter connected to the base of the second PNP transistor; 2. The voltage-current conversion circuit according to claim 1, comprising: a PNP transistor; and a fourth PNP transistor having a collector and a base commonly connected to a base of the second PNP transistor and an emitter connected to the power supply voltage. .